In recent years, a variety of approaches to optical imaging have been used for many different applications. For example, microscopes and related probes have been extensively used for research, testing and treatment of diseases or other illnesses. For invasive applications, endoscopes or other probes have been used for imaging specimen, such as for imaging tissue in anesthetized animals. For these and related applications, advancements have been made in developing relatively less-invasive imaging approaches in terms of optical components and other related equipment in order to facilitate access to tissue under one or more of a variety of conditions.
One type of optical component that has been used in applications benefiting from relatively small size is the gradient-refractive index (GRIN) lens. GRIN lenses have been used in a variety of applications, such as for imaging in photocopiers and scanners, and for microscopy and endoscopy applications. GRIN lenses are often implemented with glass rods that have a refractive index profile that is a function of the radius (or axial position) within the glass. This refractive index profile serves to focus light. For microscopy and endoscopy applications, GRIN lenses often range in diameter from a few millimeters to a few hundred microns.
Many GRIN lenses suffer from optical aberrations in one or both of the axial and lateral dimensions. These aberrations may include, for example, chromatic, spherical, astigmatism, coma, field curvature and distortion aberration. These aberrations tend to degrade the optical resolving power of GRIN lenses and prevent them from having diffraction-limited point spread functions (PSF). For example, the PSF of a 1-mm diameter doublet GRIN lens system may be in a range of about 0.9-1.2 microns in the lateral dimension, which is about 2-2.5 times the diffraction limit. For some applications, these aberrations and related characteristics are not significant or otherwise do not present issues with respect to imaging. However, GRIN lenses have been challenging to implement for a variety of applications in which such aberrations hinder the ability to form desired images, such as for fluorescence microendoscopy. Adequate resolution is often unattainable for a variety of applications needing or benefiting from a relatively high numerical aperture (NA), such as for imaging dendritic spines and mitochondria.
Difficulties associated with the above have presented challenges to imaging applications, including those involving the implementation of such micro-lens optics.